The present invention relates generally to flash photography, and more particularly to flash compensation for bounce illumination.
A flash unit outputs a controlled pulse of light in response to a trigger signal received from a camera, another flash unit, or other control device. A flash unit may be mechanically attached to a camera for primary or supplemental illumination of a photographic subject. One or more flash units may be positioned some distance away from the camera in addition to, or alternatively instead of, a flash unit mechanically attached to a camera. Each of the flash units receives a signal from the camera, or optionally from other flash units or control devices, for initiating and terminating light output. Light from the one or more flash units reflects from a photographic subject and the reflected light is collected by a camera lens for forming a photographic image. Controlling the light output from a flash unit, referred to as flash exposure control, and camera settings such as shutter speed, ISO sensitivity, and lens aperture allows a photographic image to be formed in which a range of tonal values from a photographic subject are captured in a corresponding range of tonal values. The range of tonal values in a captured image is generally smaller than the range of tonal values for the photographic subject represented in the image, and may be shifted more toward dark tones or light tones than tonal values for the subject.
A flash unit or a camera may include automatic flash exposure control for setting parameters such as flash duration, number of flashes, and other factors. Flash units with automatic flash exposure control (AFEC) may apply preprogrammed rules for controlling light output, for example by identifying the main subject in a photographic composition, by reducing the influence of very dark or very light regions on a photographic subject, or many other methods. A flash unit with AFEC may perform at least one test flash prior to a flash for making a final photographic image. The test flash may be used to account for surface reflectivity on different parts of the photographic subject and may take into account other parameters that influence flash settings used to make a final captured image. Many different algorithms have been proposed for controlling flash output to achieve different image results. For example, some flash units with AFEC determine an average tonal value for a photographic subject and adjust flash settings to render the average tonal value a predetermined gray level such as “18% gray” in the final captured image. However, if a photographic subject includes a high proportion of relatively dark areas, adjusting the flash output according to an 18% gray average tonal value may cause light areas to be overexposed in the captured image. Conversely, if the photographic subject includes a high proportion of relatively light areas, dark areas may be underexposed in the final image. Subject detail may be lost in underexposed and overexposed areas of a captured image.
A flash unit with AFEC may adjust flash output by accounting for such parameters as camera-to-subject distance, flash-to-subject distance, duration of flash pulses, and camera settings such as ISO sensitivity, lens aperture, and distance. A flash unit may receive distance information from the camera's autofocus system and use the distance information and the inverse square law for light to determine a time duration for a light pulse to be output from the flash unit to form a captured image with selected properties. This method may be applied in direct flash photography, an arrangement of camera, flash, and photographic subject in which the distance traveled by light from the flash to the subject is about the same as the camera-to-subject distance. With direct flash, a light pulse for illuminating the subject follows a path from the flash to the subject and then reflects from the subject to the camera lens, without reflecting from any intermediate surfaces between the flash and the subject. When a photographer using direct flash modifies a composition by changing the camera-to-subject distance, rearranging the photographic subject, changing camera lenses, or other changes, corresponding changes in flash settings may be predicted by using flash guide numbers, camera histograms, and other methods. For direct flash photography, there are known methods for creating a new image having tonal values comparable to a previous image when flash settings or camera settings are changed. Closely related methods permit a photographer to accurately predict new flash settings and camera settings for achieving a selected magnitude of change in captured tonal values for images created with direct flash exposures.
The larger the uncertainty in measured distances, the greater the difficulty flash units with AFEC may have in predicting correct flash settings for achieving a desired range of tonal values in a captured image. For example, in a configuration of a camera, photographic subject, and flash unit referred to as bounce flash, light output from the flash unit reflects (“bounces”) from an intermediate surface such as the ceiling in a room or a photographer's “bounce card” before illuminating a photographic subject and then reflecting toward a camera, where reflected light is captured in a photographic image. Some bounce flash units have a flash head that is rotatable relative to the camera or stand to which the flash is attached to permit the flash head to be aimed at a reflecting surface while the camera lens remains pointed at a photographic subject. Bounce flash is sometimes used to diffuse light falling on a photographic subject, creating a softening effect in the final image. Bounce flash may also be used to reposition a shadow or reflection that interferes with a desired aesthetic effect in a photograph.
When a photographic subject is illuminated by bounce flash, the length of the path traveled by light from the flash to the subject, referred to as flash-to-subject distance, may be substantially longer than camera-to-subject distance. Many guidelines have been proposed for predicting how flash settings and camera settings should be adjusted to compensate for illumination changes during bounce flash photography. However, precise compensation of bounce flash settings for a flash unit with AFEC is difficult partly because of the difficulty in measuring flash-to-subject distance, but also for other reasons such as color shifts and reflectivity of the surface from which light is bounced. Inaccurate compensation of bounce flash settings prevents accurate prediction of tonal values in the resulting image. Many photographers resort to “bracketing” flash settings recommended by an AFEC system by taking several photos, each photo representing the result of an incremental adjustment in at least one flash or camera parameter. However, bracketing may be inappropriate, distracting, or impractical depending on the photographic subject and its location. Camera histograms can be used to predict how much a change in flash settings or camera settings will affect a captured image, but it can be very difficult to associate a particular feature of a photographic subject with a particular point on a histogram plot, so predicting a change in the image of that feature can also be very difficult, especially for photographic subjects that comprise multiple tonal values for each feature of the subject. Flash units with AFEC systems which produce acceptable image results in direct flash photography may produce poor image results, for example loss of subject detail in highlight or shadow areas, when the AFEC system is allowed to control flash settings during bounce flash photography.